Never Get Fat. If You're Fat, Lose Weight.

Using data from the Framingham Heart Study, 5209 participants were followed up for 48 years from 1948. The current study however only included participants who were free from pre-existing diseases of diabetes, cardiovascular diseases and cancer. The research showed that for those who had a medium number of years lived with obesity (between five years and 14.9 years), the risk of mortality more than doubled than for those who had never been obese. The risk of mortality almost tripled for those with the longest duration of obesity (more than 15 years). Furthermore, the research showed for every additional two years lived with obesity, the risk of mortality increased by between six and seven per cent

Of course this is correlation, but I’ll weakly bet that it’s causal. Even briefly being extremely fat means you’re more likely to die (for the rest of your life). You have something like a permanent 100% increase in relative mortality risk (how likely you are to drop dead each day) if you’re fat for 7 years (but then lose the weight). After that, it’s more like 4% per year you stay fat. I don’t know how quickly the risk concentrates in the first year of getting fat, but it’s probably front-loaded. I also don’t know if yo-yo fattening/defattening is worse.

You should never get fat. Another reason not to, is that fat cells are hormonally active, and don’t retire when you lose weight; they just become less full of lipds (like an empty balloon). Supposedly an empty fat cell creates roughly as much hunger as a full one. (I don’t remember the evidence for this, but I believe it).